[docs] Fixed stable symlink not updating when changed

[bmispelon]

This was the cause of #2027 (which I resolved by changing the symlink manually at the time).

With this change the manual update is not needed anymore and the stable symlink will be updated automatically (as intended) when a new major version of Django is released.

I've tested it locally and it seems to work (it creates the symlink if it's missing, updates it if it changes, and leaves it alone otherwise). To test it:

1. Run python manage.py update_docs --lang en --force --interactive and pick "yes" for the version that's currently the default on your local install
2. Observe that the symlink was created (should be in djangodocs/en/<version>/_built/) and points to the version you just built
3. Mark another version as is_default in the admin (or manually delete/recreate the symlink to point to something else), run update_docs again and see that the symlink has changed.